hi Nick and welcome to the hq!
is that a pic of your three wheeler in your sig, looks pretty sweet
anyways as for the shee, my first recomedation would be buy a clymers manual for the banshee. I use mine all the time. As for the tank, it should fit on with relative ease. I think your right for checking to see if the frame is bent because thats what it sounds like to me. A common spot on the frame to get bent is the steering loop. this is the loop at the top of the frame that basically straddles the gas tank. Do a search and you will find pics, im horrible at explaining stuff, lol. Alot of people will re-enforce the steering loop but i have not. I feel like if my bike flips i would like the steering loop to bend over other components and if you add gussets to re-enforce it the next weak part on the bike what ever it is, will bend. The steering loop is an easy fix as well.
Check it out and get back to me, pm me or throw a post up and we can take it from there.
Peter